1. Field of the Invention
The invention relates to ships. More particularly, the invention relates to towing by means of cable. Most particularly, the invention relates to a tow cable.
2. Description of the Related Art
Marine towing includes diverse operations including recovering boats in distress, moving barges and barge trains, moving and positioning petroleum drilling platforms, transporting and positioning buoys, pulling hydrophone and other instrument assemblies, mine sweeping, underwater towing and recovery and the like. Included in vessel towing are the various operations involved in moving and also in holding a stable position, i.e. resisting motion. The dynamics of towing is different in ocean, lake and river environments and changes with weather conditions. This causes the demands of safe towing to change from towing preparations to final mooring.
In addition to the general transport of the towed vessel, towing requires compensation for static forces, dynamic forces and shock-load forces. Static forces are inertia and moment of inertia, encountered during turning which cause a towed vessel to resist motion.
Dynamic forces occur when the towed vessel is moving. These forces are caused by the towing vessel, and the effects of waves and wind. These forces are based on towed vessel characteristics, including shape, displacement, arrangement and rigging. Friction forces vary with hull shape. Greater wetted surface area causes greater frictional resistance. Hull appendages such as propellers, shafts, skegs, keel and rudders contribute to wetted surface area and frictional resistance. Frictional resistance is managed with towing speed. Higher towing speed causes higher friction resistance and more strain on the tow rigging. Form drag plays a large role in the ability to control changes in the towed vessel's movement. Different hull shapes react to motion through the water in different ways. The shape and size of the towed vessel's hull can either help or hinder effort to move in a straight line, when changing heading, and motion changes in response to waves due to buoyancy. The less water a hull shape has to push out of its way, the easier it will move through the water. A deep draft full-hulled vessel takes more effort to move than one with a fine, hallow hull. A large amount of lateral resistance, spread evenly over the length of the hull, hinders the effort to change a towed vessel's direction. A towed vessel may be able to help offset form drag by using its rudder. A surface wave forms at the bow while the hull moves through the water. Size of the bow wave increases as vessel speed increases, causing the wave to resist the bow moving through the water.
Shock load is rapid, extreme increase in tension on the tow cable, which transfers through the tow rig and fittings to both vessels. The frictional forces of wave drag, spray drag and wind drag act on the hull, topsides, and superstructure and rigging. They all have a major effect on the motion of the towed vessel, and the transfer of forces to and through the towing rig. These constantly changing forces vary with the towed vessel's motion relative to the environmental elements and are directly related to the towed vessel's exposure to them. These forces can add up and cause shock-loading. Wind and wave drag also cause a distressed drifting vessel to make leeway, that is, motion in a downwind direction.
A towed vessel is rarely under the influence of only one force. Usually a combination of forces is experienced, making the tow more complex. Some individual forces are very large and relatively constant. These are relatively easier to handle provided that all towing force changes are gradual. When forces change in an irregular manner, tension on the tow rig varies. Shock-loading may cause severe damage to both towing and towed vessel and overload a tow rig to the point of tow cable or bridle failure. Shock-loading can cause momentary loss of directional control by either vessel and has the potential of capsize small vessels.
Even in calm winds and seas, a towing vessel can encounter a large amount of frictional resistance from form and wave drag when towing a large fishing vessel with trawl lines and net still in the water. The tow rig and vessel fittings can be under heavy strain and the tow vessel engine loads rather high. If the net catches on an obstacle, this new load acts through the tow rig and can suddenly increase stress to a potentially damaging amount. This shock-load can part the tow cable or destroy fittings.
A longer tow cable reduces the effect of shock-loading in two ways. The weight of the line causes a dip in the line, known as a catenary. The longer the cable the greater the possible catenary. When tension increases, energy from shock-loading is dissipated by reducing catenary before it is transferred through the rest of the rig and fittings. A second benefit of a longer tow cable is additional cable length from stretching. Depending on the type of tow cable, lengthening the cable by 50 feet gives 5 to 20 feet more stretch length. This stretching absorbs shock-load. Lengthening the tow cable can be used to keep the two vessels in step and to reduce shock-load.
The effect of shock-loading can be mitigated by tacking to either side of the actual desired course rather than setting a course directly into or directly down heavy seas. This is accomplished by keeping the seas 30° to 45° either side of dead ahead or dead astern. A drogue can be attached to the towed vessel to help prevent it from accelerating down the face of a wave. A drogue adds form drag, but may reduce shock-loading. Shock-loading can capsize or swamp the towed vessel. The additional towing force from a shock-loaded tow cable may cause a smaller vessel to climb its bow wave and become unstable or it may pull the bow through a cresting wave.
In heavy seas, towing vessel speed can be adjusted to match that of the towed vessel. This requires constant observation of the towed vessel and changing speed to compensate for the approaching or receding seas on the towed vessel. One serious danger is cable snap-back. This can occur when the tow cable is stretched to breaking. Some nylon cordage can stretch up to 40% of its length before parting.
Another condition encountered during towing is a persistent induced vibration in the tow cable referred to as strum. This vibration is transmitted into a towed instrument array and can cause damage to instrument components. The vibration can be reduced by changing the length of tow cable so that the cable length is not a harmonic multiple of the vibration. Strum is more fully described in U.S. Pat. No. 6,494,158 to A. A. Ruffa for a Method For Reducing Strum In Tow Cable, incorporated herein by reference.
There is a need in the art of marine towing for an improvement that helps to avoid or reduce inherent dangers. Risk and associated insurance rates may be reduced with an improved tow cable.